U.S. patent number 5,764,605 [Application Number 08/571,952] was granted by the patent office on 1998-06-09 for g factor alignment.
This patent grant is currently assigned to Deutsche Thomson-Brandt GmbH. Invention is credited to Christian Buchler, Friedhelm Zucker.
United States Patent |
5,764,605 |
Zucker , et al. |
June 9, 1998 |
**Please see images for:
( Certificate of Correction ) ** |
G factor alignment
Abstract
A system for guidance of an optical scanning device is disclosed
in which a gain factor (G factor) is responsive to a track error
signal which is a function of the deflection of a scanning device.
Deviation of the G factor setting from an optimum setting is
automatically determined in conjunction with an open track control
loop by driving a servo device (SV) to deflect an actuator from its
neutral position and by evaluating a push-pull signal (PPTE). The
G-factor is automatically set to an optimum value.
Inventors: |
Zucker; Friedhelm
(Villingen-Schwenningen, DE), Buchler; Christian
(Villingen-Schwenningen, DE) |
Assignee: |
Deutsche Thomson-Brandt GmbH
(DE)
|
Family
ID: |
6492449 |
Appl.
No.: |
08/571,952 |
Filed: |
June 17, 1996 |
PCT
Filed: |
June 30, 1994 |
PCT No.: |
PCT/EP94/02146 |
371
Date: |
June 17, 1996 |
102(e)
Date: |
June 17, 1996 |
PCT
Pub. No.: |
WO95/02244 |
PCT
Pub. Date: |
January 19, 1995 |
Foreign Application Priority Data
|
|
|
|
|
Jul 10, 1993 [DE] |
|
|
43 23 067.9 |
|
Current U.S.
Class: |
369/44.29;
369/44.35; 369/44.36; G9B/7.089; G9B/7.066; G9B/7.093;
G9B/7.091 |
Current CPC
Class: |
G11B
7/094 (20130101); G11B 7/0945 (20130101); G11B
7/0901 (20130101); G11B 7/0941 (20130101) |
Current International
Class: |
G11B
7/09 (20060101); G11B 007/09 () |
Field of
Search: |
;369/44.35,44.36,54,44.13,44.25,44.29 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
0264837 |
|
Apr 1988 |
|
EP |
|
0274031 |
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Jul 1988 |
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EP |
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0478314 |
|
Apr 1992 |
|
EP |
|
4029040 |
|
Mar 1991 |
|
DE |
|
2-185723 |
|
Jul 1990 |
|
JP |
|
90/08381 |
|
Jul 1990 |
|
WO |
|
91/08568 |
|
Jun 1991 |
|
WO |
|
Other References
Patent Abstracts of Japan, vol. 14, No. 465, and Japan Pat. No.
2-185723..
|
Primary Examiner: Tran; Thang V.
Attorney, Agent or Firm: Tripoli; Joseph S. Kurdyla; Ronald
H. Burke; Alexander J.
Claims
We claim:
1. Method for setting a gain (a G factor) altering a slope of a
track error signal, said track error signal being a composite
signal comprised of,
a first component dependent on position of an actuator relative to
a track and
a second component associated with said track error signal slope,
said second component being dependent on deflection of said
actuator from a neutral position and being correctable by means of
said G factor, characterized in that
deviations in G factor setting from an optimum G factor setting are
reduced in an automated manner in conjunction with an open track
control loop by
driving a servo device to deflect said actuator away from said
neutral position,
(b) deriving a correction signal from a push-pull signal resulting
from driving said servo device and
(c) setting said G factor in an automated manner in response to
said correction signal.
2. Method according to claim 1, further including the steps of
measuring a mean value of said push-pull signal during deflection,
and
updating said G factor in an iterative manner to reduce said mean
value.
3. Method according to claim 1, further including the steps of
driving said servo device with a prescribed oscillator signal,
and
deriving said correction signal from a signal formed by a
synchronous detector from said push-pull signal during
deflection.
4. Method according to claim 3, wherein the signal formed by the
synchronous detector is an integration signal.
5. Method according to claim 3, wherein the signal formed by the
synchronous detector is
a signal including both integral and proportional components.
6. Method according to claim 1, characterized in that
said deviations are reduced in an automated manner after the
insertion of a recording medium into a device for playing back or
recording information.
7. Apparatus for setting a gain (a G factor) altering a slope of a
track error signal, said track error signal being a composite
signal comprised of,
a first component dependent on position of an actuator relative to
a track and
a second component associated with said track error signal slope,
said second component being dependent on deflection of said
actuator from a neutral position and being correctable by means of
said G factor, characterized in that
deviations of G factor setting from an optimum G factor setting are
automatically corrected by
a servo device connected to a control device for deflecting said
actuator away from said neutral position,
an evaluation unit for processing a push-pull signal during
deflection and producing a correction signal, and
a setting device connected to said evaluation unit for automated
setting of said G factor in response to said correction signal.
8. Apparatus according to claim 7, characterized in that
said control device for deflecting said actuator from said neutral
position is a microprocessor, and said control device is coupled to
a low-pass filter and an analog-to-digital converter to form said
evaluation unit, and said evaluation unit is connected to a
preamplifier providing said push-pull signal for automated setting
of said G factor by said setting device.
9. Apparatus according to claim 7, characterized in that
said control device is an oscillator,
said evaluation unit is a synchronous detector, and
said setting device is a sample-and-hold circuit.
10. Apparatus according to claim 7, characterized in that said
apparatus for automatically reducing deviations of G factor is
included within a device for playing back or recording information.
Description
The invention relates to methods and arrangements for G factor
alignment or for setting a device, providing control signals or a
track error signal in the case of deviations of the scanning beam
from the information track of a recording substrate, in control
loops for track guidance of optical scanning devices which for the
purpose of track guidance of the scanning device make use, for
example, of the known transverse push-pull or differential
push-pull methods. Such scanning devices are fitted, for example,
to CD players, videodisc players, draw disc players and
magnetooptical recording and playback devices.
Push-pull refers in general to a method for generating a track
error signal for the track control loop in the case of radial or
lateral deviation of the scanning beam from the information track
of the recording substrate, the push-pull signal being the
differential signal of an at least bipartite photodetector which
permits the scanning device or the scanning beam to be guided on
the track of the recording substrate. The light reflected by the
optical recording substrate and detected by the photodetector has
an intensity distribution, which is a function of the nature of the
incidence of the scanning beam on an information track or of the
track positioning, is converted into electric signals and used as
reference input variable or push-pull signal in the track control
loop, which keeps the scanning beam on the information track. For
this purpose, a track control amplifier or track controller
controls a scanning device or a so-called pickup. The pickup
contains a laser which generates the scanning beam, a movable
holder, referred to as an actuator, of an objective lens for fine
positioning and focusing of the scanning beam, which is controlled
by a servo device, a beam splitter for splitting the transmitting
and receiving directions of the light, and a photodetector for
evaluating the light reflected by the recording substrate, cf.
Electronic Components & Applications, Vol. 6, No. 4, 1984,
pages 209 to 215. In order to be able to scan the relatively large
track area of a recording substrate, the pickup is arranged as a
rule on a coarse drive which can be moved perpendicularly to the
information track and forms with the servo device the so-called
radial drive for track guidance of the scanning device. The servo
device, also referred to as the vernier drive, is provided in this
case particularly in order to be able to follow rapid track
displacements owing to disc runout or eccentricities of the
recording substrate. Because of disc runout or eccentricity of the
recording substrate, the objective lens is in perpetual movement
via the pickup in order to follow the information track of the
recording substrate. The optical axis of the scanning device, which
is a function of the position of the objective lens varies its
position and wanders over the photodetector, and the light
reflected by the recording substrate reaches the photodetector as a
function of the current position of the actuator. The result of
this is that despite ideal positioning of the scanning beam on the
information track of the recording substrate a track error signal
or push-pull signal is generated owing to inclination and movement
of the actuator and a deviation from the information track is
simulated. Furthermore, the push-pull signal can simulate track
deviations which originate from the mechanical stability or
instability of the pickup and its adjustment. The photodetectors
are fastened to the pickup by means of an adhesive, for example,
and float in the adhesive as a function of temperature and air
humidity, as a result of which movements are caused. It is
therefore not possible to distinguish by means of the push-pull
signal whether the track error signal is actually to be ascribed to
a track deviation, an inclination of the actuator, instabilities in
the pickup or else to properties of the recording substrate. The
so-called transverse push-pull and the differential push-pull are
already known for the purpose of avoiding this disadvantage, cf. WO
90 08 381 and Kiyoshi Ohsato, Differential push-pull method,
Optical Memory Symposium, Japan, 18.2.1986. In the transverse
push-pull or TPP, a single-beam scanning method, a photodetector is
used which comprises four quadrants and which is assigned the
actuator in such a way that in the case of track deviation the
reflected light beam does not move parallel to a dividing line of
the four-quadrant photodetector.
As a result, in the case of a deflection signal components are
available in two mutually perpendicular directions, which render it
possible to use a correction factor, the so-called G factor, to
obtain a track error signal, also referred to as the track error,
which is to be ascribed exclusively to track deviations or track
errors. However, because of the numerous influencing factors which
originate from the scanning device and also from the recording
substrate, it can happen that a calculated value does not
correspond to the conditions specifically present, with the result
that the G factor must be set manually for each device. The problem
of setting the G factor, or the G factor alignment also occurs in
the case of the so-called differential push-pull. The differential
push-pull or DPP is a three-beam method, that is to say in addition
to a main beam positioned on the track two auxiliary beams provided
between the tracks ahead of and behind the main beam are used for
track guidance, and the spots provided by the beams wander in the
same direction in the case of deflection of the actuator. The
light, reflected by the recording substrate, of the main beam and
of the auxiliary or secondary beams is respectively detected with a
bipartite photodetector, and a push-pull signal is provided from
each photodetector by means of a differential amplifier. Owing to
the fact that the main beam is positioned on the middle of the
track and the auxiliary beams are positioned exactly between the
tracks, the push-pull signal of the main beam detector is inverse
with respect to that of the auxiliary beam detectors. Track error
signals simulated, in particular, by actuator movements are
eliminated owing to the fact that the push-pull signals of the
auxiliary beam detectors are subtracted, in a manner added to and
multiplied by a G factor, from the push-pull signal of the main
beam detector. Owing to the numerous influencing factors, which
have the effect that a track error signal is generated despite
ideal track positioning and that the intensity distributions of the
main and auxiliary beams can be different, it is necessary here, as
well, for the G factor to be set or aligned individually manually
and for each device. In order to set the optimum G factor, which is
to be set independently of the loop gain of the track guidance
control loop, use is made of a critical recording substrate or a
recording substrate with disc runout or eccentricity in the form of
a measuring disc which deflects the actuator and generates a
corresponding push-pull track error signal. An oscilloscope is then
used to observe the characteristic of the track error signal in the
case of an open track control loop, and the G factor is optimally
set. For this purpose, the setting of a G factor setter is varied
until the track error signal represented on the oscilloscope occurs
in a range which is optimum for control. As a result, control
signals to be ascribed in essence to deviations of the scanning
beam from the track are provided, and influences on the push-pull
track error signal which are specific to the device and to the
recording substrate are compensated. This is performed manually,
purely by testing, and is therefore very time-consuming and must be
carried out individually for each pickup or device on the basis of
manufacturing tolerances. It is, furthermore, disadvantageous that
the device parameters vary owing to ageing and external influences,
with the result that an optimum G factor setting can be ensured
only for a relatively short period. Moreover, no account is taken
of influences on the track error signal which originate from
different recording substrates. Independently of the G factor by
means of which it is achieved that the control range of the track
control loop can be fully utilized even when the actuator is
deflected or other influences falsify the error signal, there is a
need to compensate, an offset which may be present in the track
control loop by means of a method described, for example, in EP 02
74 031 B1, for example, an offset which leads to asymmetry in the
control range. However, the compensation of an offset voltage in
the track control loop is referred only to a defined actuator
position, with the result that despite offset compensation the
control range is restricted or control stops in the case of
non-optimum setting of the G factor and deflection of the actuator.
There is therefore generally an additional requirement to set an
optimum G factor or to align the G factor. By contrast with setting
the control loop gain by means of which the proportional gain in
the closed control loop is aligned, if necessary in an automated
manner, compare EP 02 64 837 B1, the G factor setting is performed
with an open control loop and is a precondition for the
effectiveness of the control loop in the entire control range.
SUMMARY OF THE INVENTION
It is therefore the object of the invention to specify a method and
an arrangement by means of which the outlay required for setting an
optimum G factor is reduced and yet both influences specific to the
device and influences specific to the recording substrate are taken
into account in the optimum G factor setting.
This object is achieved by means of the features specified in
claims 1 and 7. Advantageous developments are specified in the
subclaims.
The principle of the invention consists in that deviations in the G
factor setting from an optimum G factor setting are determined in
an automated manner in conjunction with an open track control loop
by driving the servo device to deflect the actuator from its track
position and evaluating the push-pull signal, and the G factor is
set in an automated manner to an optimum G factor. Two different
approaches to a solution were found for this purpose, which consist
in that in conjunction with an open track control loop the actuator
is deflected by driving its servo unit and for the purpose of
determining the deviation of a current G factor setting from an
optimum G factor setting either a mean value of the push-pull
signal is measured and the optimum G factor is set optimally by
means of an iterative method in an automated manner, or a
synchronous detector is used to form from the push-pull signal
during the deflection of the actuator a signal which is suitable
for automated setting of an optimum G factor. These methods can be
applied both in conjunction with the known transverse push-pull and
in conjunction with the known differential push-pull. The signal,
formed using a synchronous detector from the push-pull signal
during the deflection of the actuator, for automated setting of an
optimum G factor is preferably an integration signal or a signal
including both integral and proportional components. The advantages
of using a signal including both integral and proportional
components consist in that the risk of a connected integrator
moving as far as into the boundary is reduced and operation is by
means of a fixed residual error. Furthermore, the proportional
component has an advantageous influence on the operating state in
which no input signal is available.
The method based on averaging the track error signal and iteration
is realized by means of a microprocessor, it being the case,
advantageously, that it is possible to use a microprocessor which
is generally present in such devices. This solution requires a low
outlay on circuitry and owing to the implantation in the processor
is highly flexible with respect to the use of different iteration
principles.
The integrative method is determined by the use of a synchronous
detector, the latter already directly detecting the direction of
the deviation, with the result that the setting direction
fundamentally does not vary.
The common aim of the methods consists in the push-pull track error
signal being uniform over the entire range of the deflection of the
actuator and not being influenced by other noise quantities, since
this forms the basis of an optimum G factor setting. Only in the
case of a uniform track error signal in the deflection range of the
actuator, an offset correction already being assumed, can the full
effectiveness of the track control loop be ensured. As already
mentioned, in the case of the iterative method the mean value of
the push-pull signal is measured during the deflection of the
actuator, and the optimum G factor is set in an iterative method.
For this purpose, it is preferred to connect a low-pass filter at
an output providing the push-pull track error signal, in order to
form the mean value, and the low-pass filter is connected via an
analog-to-digital converter to a microprocessor for the purpose of
evaluating the mean value of the push-pull signal in the case of
deflection of the actuator. The track control loop is opened by
means of the microprocessor, and a G-factor setter G is influenced
stepwise and in terms of direction until upon deflection of the
actuator the mean value of the track error signal exhibits no rise
in the mathematical sense, that is to say neither an increase nor a
decrease. A precondition for the alignment is the insertion in the
device of a recording substrate which, by contrast with the known
method, can be an arbitrary recording substrate. As a result, there
is no longer a need to provide special measuring discs which
represent a relatively high cost factor. The servo device is driven
by the microprocessor in order to deflect the actuator. The method
can be carried out using externally arranged means, it being the
case, however, that because the recording substrate used also
influences the G factor it is particularly advantageous to provide
the means required for the automated G factor alignment in the
device and to undertake in an automated manner optimum setting of
the G factor with each renewed insertion of a recording substrate
and/or also in pauses during the playback operation.
The integrative method can also be realized in an automated manner
both using external means and, preferably, in the device without
manual alignment. In order to realize the integrative method, a
synchronous detector is connected via changeover switches to an
output which provides the push-pull track error signal. The
synchronous detector provides a signal, including integral or
proportional and integral components, for influencing a G factor
setter for optimum setting. In order to deflect the actuator, the
servo device is separated from the track control amplifier and
connected to an oscillator which provides an appropriate control
signal for deflecting the actuator. Also connected to the
oscillator is a trigger by means of which changeover switches at
the input of the synchronous detector are controlled. The direction
into which the G factor setter is to be influenced for optimum
setting is determined directly thereby. The determination of the
size or of the value of the optimum setting of the G factor is
performed by integrating the remaining deviations from the optimum
characteristic of the track error signal in conjunction with
continuous approximation to the optimum G factor setting with the
synchronous detector. The analog output signal of the synchronous
detector is preferably digitized and fed to a sample-and-hold
circuit, in order to ensure a high long-term stability of the
determined value of the optimum G factor setting. After the
alignment of the G factor, the track control loop is closed and the
normal operation of the device is initiated. A microprocessor
present in the device is preferably used to control this
sequence.
Fundamentally, for the purpose of automated determination of
deviations of the G factor setting from an optimum G factor setting
the servo device is connected to a control device for deflecting
the actuator from its track position, an evaluation unit which
assesses the push-pull signal during the deflection is provided,
and a setting device for automated setting of a G factor setter to
an optimum G factor is connected to the evaluation unit. The
control device for deflecting the actuator from its track position
is preferably formed either by a microprocessor or by an
oscillator, and the evaluation unit is preferably either a low-pass
filter having a microprocessor, connected via an analog-to-digital
converter, or a synchronous detector downstream of which a
sample-and-hold circuit is connected, and the arrangement for
automated determination and automated setting of an optimum G
factor is preferably arranged in the device for playing back and/or
recording information.
The solutions specified can be used to carry out the optimum
setting of the G factor advantageously in an automated manner, with
the result that there is no longer a need for manual setting. The
automated G factor setting can, moreover, be carried out in the
replay and/or recording device, as a result of which an appropriate
account is taken in accordance with the current conditions of
influences of the recording substrate on the G factor and of
changes in the device parameters.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in more detail below in exemplary
embodiments with the aid of drawings, in which
FIGS. 1(a-c) shows track error signal diagrams,
FIG. 2 shows a schematic sketch of a photodetector for transverse
push-pull,
FIG. 3 shows a schematic sketch of a photodetector for differential
push-pull,
FIG. 4 shows a schematic sketch of a circuit arrangement for
iterative G factor alignment,
FIG. 5 shows signal characteristic diagrams for automated G factor
setting by means of a synchronous detector, and
FIG. 6 shows a schematic sketch of a circuit arrangement for G
factor alignment by means of a synchronous detector.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 specifies track error signal diagrams which represent
different track error signals TE as a function of the path W of the
deflection of the actuator. The track error signal TE represented
in FIG. 1a characterizes both an offset in the control loop owing
to the asymmetry relative to the W-axis of the diagram, and a G
factor which is not optimally set owing to the rise or fall in the
case of deflection of the actuator. The known correction of the
offset leads to the track error signal diagram which is represented
in FIG. 1b and which, despite the characteristic through the centre
of the coordinate system, cannot yet be regarded as optimum since,
in accordance with FIG. 1b, in the case of an unfavourable G factor
and deflection of the actuator by a path W the track control is not
effective to the full extent, or even ceases to function. Specified
in FIG. 1b are an operating point AP0 in the middle range of the
track error signal TE and, in accordance with this operating point
AP0, limiting operating points AP1, AP2 at the minimum and maximum,
respectively, of the track error signal range in the case of
deflection of the actuator by a path W. It is to be seen that
because of the rise in the track error signal TE upon deflection of
the actuator the control range of the track error control is
restricted at one end, and that upon overshooting a path W
corresponding to the limiting operating point AP2 the control
becomes ineffective. The G factor is generally to be set optimally,
in order to ensure the effectiveness of the track control loop to
the full extent even in the case of deflections of the actuator.
Owing both to unavoidable eccentricities and to the subdivision of
the radial drive into coarse drive and vernier drive, the
deflection of the actuator occurs basically in the playback and/or
recording modes of an optical recording substrate. The G factor is
regarded as being optimally set when the track error signal TE has
a characteristic, corresponding to FIG. 1c, which is symmetrical
relative to the W-axis and has no rise. This applies both to a
track control based on the transverse push-pull method and to one
based on the differential push-pull method.
In the transverse push-pull or TPP, a photodetector comprising four
quadrants A, B, C, D in accordance with the schematic sketch
represented in FIG. 2 is arranged in such a way that the path W of
the scanning beam does not extend parallel to a dividing line of
the quadrants A, B, C, D of the photodetector.
Consequently, deflection of the actuator leads to variation in the
detected light quantity in mutually perpendicular directions, and
permits a correction factor, the so-called G factor, to be used to
separate the track error signal TE which is to be ascribed
exclusively to track deviations. It is possible using the
coordinate system X, Y specified in FIG. 2 to set up the
relationships X=(A+D)-(B+C) and Y=(A+B)-(D+C) and to specify
TE=X-G*Y as track error signal. The G corresponds in this case to
the correction value or the so-called G factor which is to be
optimally set.
In the differential push-pull or DPP, which operates with one main
and two auxiliary scanning beams as well as three bipartite
photodetectors D1, D2, D3, in order to generate a track error
signal TE to be ascribed exclusively to track deviations in
accordance with FIG. 3 the signals generated by means of
photodetectors D1, D3 of the auxiliary beams are firstly added by
means of a summer S and then corrected with a G factor, and the
signal detected by the photodetector of the main scanning beam is
then subtracted from the result by means of a subtractor M.
In order to be able to set the G factor optimally in an automated
manner, two methods are proposed which can be applied in
conjunction both with TPP and with DPP. Common to these methods is
that deviations of the G factor setting from an optimum G factor
setting in conjunction with an open track control loop are
determined in an automated manner by driving the servo device to
deflect the actuator from its track position and evaluating the
push-pull signal and the G factor is set to an optimum G factor in
an automated manner. For this purpose, either a mean value of a
push-pull signal is measured during the deflection and the G factor
is set in an iterative method by means of a microprocessor .mu.P
until the mean values correspond in the case of different
deflections, or the actuator is deflected from its track position
by means of a prescribed oscillator signal and the G factor is
determined and set by means of a signal formed by a synchronous
detector from a push-pull signal during the deflection. The
foundation of the automated G factor setting is therefore formed by
an iterative and/or a predominantly integrative method, it being
preferable that proportional components should also be included in
the integrative method. For the purpose of an optimum mode of
operation of the track control loop, both methods for G factor
setting assume that in addition to the G factor setting correction
of the offset is provided in a known way in the controlled system.
In order to permit an optimum G factor alignment independently of
external influences and taking account of the influences
originating from the recording substrate, the automated G factor
alignment is preferably carried out in the device after the
insertion of a recording substrate into the device for playing-back
and/or recording information and/or during pauses in the playback
of information.
For the purpose of G factor setting in accordance with FIG. 4 and
FIG. 6, the push-pull signal or track error signal detected during
deflection of the actuator is fed to a G factor setter G which is
preferably formed by an amplifier PREA which preferably has in at
least one feedback branch for setting the G factor a resistor whose
value can be varied electrically. A push-pull track error signal
PPTE which in conjunction with an open track control loop forms the
starting point of the methods for automated G factor setting is
then available at the output of the G factor setter or of the
amplifier PREA before or after an offset correction OFFSET.
In the iterative method, the push-pull track error signal PPTE is
fed, in accordance with FIG. 4 and for the purpose of averaging, to
a low-pass filter which is formed by a resistor R and a capacitor
C. The low-pass-filtered push-pull track error signal PPTE is
tapped at the connecting point between the resistor R and the
capacitor C and fed to a microprocessor .mu.P via an
analog-to-digital converter AD. It is preferred to use as the
microprocessor .mu.P a microprocessor which is generally present in
an optical playback and/or recording device. The first step for the
G factor alignment is to open the track control loop of the device
after the insertion of a recording substrate and/or also during a
pause in playback by means of the microprocessor .mu.P, and to
deflect the actuator from its rest position or neutral position. In
accordance with FIG. 4, for this purpose the connection between the
track control amplifier TRV and the servo device SV of the device
is broken, and a control signal for deflecting the actuator or
pickup PU is fed to the servodevice SV from the microprocessor
.mu.P. The push-pull signal detected by means of the photodetector
of the pickup PU is then fed, in a manner analogous to the normal
operation of the device, to the amplifier PREA which contains (in a
way not represented) a G factor setter. The push-pull track error
signal PPTE corresponding to the deflection of the actuator by a
path W is then in turn evaluated after averaging by means of the
microprocessor .mu.P. This evaluation by means of the
microprocessor .mu.P includes, in particular, an investigation of
whether the characteristic of the track error signal TE has a rise
or fall. If this applies, the current value of the G factor setting
is varied by means of the microprocessor .mu.P and the
characteristic of the track error signal TE is once again analysed
with regard to a deviation from an optimum setting of the G factor.
This process is then repeated in an automated manner in an
iterative method as far as the optimum setting of the G factor.
Owing to the automation of the alignment, the alignment can be
carried out in an advantageous way inside the device without the
connection of external measuring devices and without manual outlay
on alignment. After the optimum setting of the G factor, the
connection between the track control amplifier TRV, to which an
optimum track error signal TE is now fed for control, and the servo
device SV is then reestablished by means of the microprocessor
.mu.P, and an optimum mode of operation of the device is ensured.
As an additional feature by comparison with known circuit
arrangements of comparable devices, the circuit arrangement,
described with the aid of this function, for carrying out the
method in accordance with FIG. 4 has essentially only one resistor
R and one capacitor C for averaging the track control signal TE and
push-pull track error signal PPTE by means of a low-pass filter,
with the result that the outlay on realizing the method and the
circuit arrangement is very low.
The predominantly integrative method is explained with the aid of
FIGS. 5 and 6. The block diagram given in FIG. 6 for carrying out
the predominantly integrative method includes an information medium
CD which is scanned by the scanning beam of a laser diode LD, the
light of the laser diode LD passing a beam splitter STT and an
objective lens OL for focusing the scanning beam on the information
medium CD and for detection by means of a photodetector PH which
includes four quadrants A, B, C, D. It is evident with the aid of
the four-quadrant detector that this exemplary embodiment was
designed for the transverse push-pull. It should therefore be
pointed out once again that the application is possible in an
equivalent way in conjunction with the differential push-pull. The
light intensity signals detected by means of the quadrants A, B, C,
D of the photodetector PH are fed, corresponding to the statements
of the TPP, to a first and a second preamplifier PR1, PR2, whose
outputs are connected via one resistor R1, R2 in each case to an
input of an amplifier PREA. The non-inverting input of the
amplifier PREA is led to earth via a third resistor R3, and the
inverting input is connected to the output of the amplifier PREA
via a resistor which can preferably be set by means of an electric
signal. The amplifier PREA connected in this way forms the actual G
factor setter G, and there is provided at its output an adder
OFFSET for the known correction of an offset in the control loop,
with the result that there is available downstream of the adder
OFFSET a push-pull track error signal PPTE which is to be set
optimally with regard to the G factor, in order to be able to be
used as controlled variable for a connected track control amplifier
TRV. As first step towards the G factor setting, the track control
loop amplifier TRV is separated from the servo device SV, which is
provided for deflecting the objective lens OL, by means of a switch
S1, and the servo device SV is connected to an oscillator OSZ. The
changeover signal is provided in this case by a microprocessor (not
represented) present in the device, and the oscillator OSZ
preferably provides a sinusoidal control signal for deflecting the
actuator or the objective lens OL. The control signal deflects the
actuator or the objective lens OL by a path W represented in FIG. 5
and, in accordance with FIG. 6, a trigger SWS by means of which a
rectangular-pulse signal SWS represented in FIG. 5 is formed for
the purpose of driving two changeover switches S2, S3 is connected
at the oscillator OSZ. The push-pull track error signal PPTE or
frame potential is alternately applied by means of the changeover
switches S2, S3 at the inputs of a differential amplifier via a
resistor R3 or a resistor R4, and the noninverting input of the
differential amplifier is connected via a first capacitor C1 to a
frame terminal, while its noninverting input is connected via a
second capacitor C2 to the output of the differential amplifier.
Together with the connected differential amplifier, the changeover
switches S2, S3 form a so-called synchronous detector SDV by means
of which the synchronous detector output signal SDVA, represented
in FIG. 5, of a push-pull track error signal PPTE1 or PPTE2, is
formed in accordance with the initial setting of the G factor
setter G. Since in conjunction with the changeover owing to the
signal of the oscillator OSZ which deflects via the path W the
direction in which the G factor setter G is to be controlled for
the purpose of optimum setting is determined at the input of the
synchronous detector SDV, deviations of the push-pull track error
signal PPTE1 or PPTE2 from an optimum characteristic can be used
directly as setting criterion. The synchronous detector SDV is
preferably a differential synchronous integrator or synchronous
demodulator by means of which as a consequence of knowledge of the
deflection direction of the objective lens OL there is
advantageously a direct determination of the direction, that is to
say decreasing or increasing, in which the G factor is to be
influenced for optimum setting. No iteration is required. Since the
frequency at which the objective lens OL is deflected is known on
the basis of the driving by the oscillator OSZ, the direction in
which the G factor setter G is to be controlled is determined
directly, because it is to be assumed that the G factor or the
amplification of the amplifier PREA is too large when the
oscillator signal and the synchronous demodulator signal are in
phase and, on the other hand, that the G factor or the gain of the
amplifier PREA is too low when the oscillator signal and
synchronous demodulator signal are in antiphase. The oscillator
frequency is preferably a frequency below the mechanical
self-resonance frequency of the actuator. In accordance with FIG.
5, a synchronous detector input signal SDVE corresponding to the
deviation is active at the input of the synchronous detector SDV,
and a sample-and-hold circuit SAH for driving the G factor setter G
is connected, preferably via an analog-to-digital converter AD at
the output of the synchronous detector SDV for the purpose of
automated setting of the G factor. The sample-and-hold circuit SAH
is used in a known way respectively to accept the current values
for setting the G factor setter G and, finally, the optimum setting
value is retained. This control and the changeover into closed-loop
control are preferably carried out by means of the microprocessor
already mentioned. In order to ensure a high long term stability of
the setting value, determined in an automated manner, for the
optimum G factor alignment, a digital sample-and-hold circuit SAH
was preferably provided, although it would also be possible in
principle to use a sample-and-hold circuit SAH operating in an
analog manner. Furthermore, the exemplary embodiment has been
described with the aid of a synchronous detector SDV which
essentially provides integral signal components, although the use
of a synchronous detector SDV which also provides proportional
signal components is to be preferred, in particular in conjunction
with the alignment response in the case of slight deviations from
the optimum setting. The advantages of using a signal including
both integral and proportional components consist in that the risk
that a connected integrator moving as far as into the boundary is
reduced, and operation is by means of a fixed residual error.
Furthermore, the proportional component has an advantageous
influence on the operating state in which no input signal is
available.
The exemplary embodiment was selected, in particular, from the
point of view of explaining the principle, as a result of which the
scope of application is not restricted in conjunction with other
synchronous detectors SDV.
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